1. Field of the Invention
The present invention relates to a photographic optical system and an image pickup apparatus equipped with the photographic optical system, and is suitable for, for example, video cameras, digital still cameras, TV cameras, monitoring cameras, and silver-halide film cameras which use solid-state image sensors.
2. Description of the Related Art
As a photographic optical system for a long focal length, a telephoto type photographic optical system including a front lens unit having positive refractive power and a rear lens unit having negative refractive power in order from an object side to an image side is known. Here, a long focal length refers to a focal length long, for example, as compared with the size of an effective imaging range. In general, in a telephoto lens having a long focal length, as the focal length extends, chromatic aberrations such as, particularly, axial chromatic aberration and chromatic aberration of magnification occur among various aberrations.
Japanese Patent Application Laid-Open No. 9-145996 discusses a telephoto lens where aberration is corrected (removed) by combining a positive lens using a low-dispersion material, such as fluorite or a product named S-FPL51 (a product of OHARA INC.), which has anomalous partial dispersion, with a negative lens using a high-dispersion material. The telephoto lens has a focal length of 294 mm to 392 mm and an F-number of approximately 4.08 to 5.6.
In a telephoto lens having a small F-number, as the F-number decreases, spherical aberration and coma occur frequently in particular among various aberrations. Thus, Japanese Patent Application Laid-Open No. 8-327897 discusses a telephoto lens which increases a degree of freedom of aberration correction by increasing the number of lens elements, to correct spherical aberration or coma in a telephoto lens having a small F-number. The telephoto lens has a large relative aperture and has a focal length of 294 mm to 588 mm and an F-number of approximately 2.88 to 4.08.
Meanwhile, as a method of lightening a weight of a lens while correcting various aberrations including chromatic aberration of an optical system, a method of using a diffractive optical element where a diffractive optical unit having a diffraction function is mounted on a substrate, on a lens surface or a portion of an optical system is known. Japanese Patent Application Laid-Open No. 2009-271354 discusses an optical system where an overall lens weight is lightened by shortening an entire lens length while correcting chromatic aberration or forming a lens made of a glass material whose specific gravity is relatively low through the method. The optical system is a telephoto lens of a large relative aperture having a focal length of 293 mm to 391 mm and an F-number of approximately 2.9 to 4.1. The optical system corrects aberration of a mono color with an aspheric surface by an optical power of a first lens unit increased by shortening an entire lens length and corrects chromatic aberration with the diffractive optical element. Accordingly, correction of various aberrations and small-scale size and light weight of the system are promoted.
In addition, in many photographic lenses (optical systems), focusing from an infinitely distant object to a close object is performed by moving an entire photographic lens or some lens units of a photographic lens. Among them, in the case of a telephoto lens having a long focal length, as the entire lens becomes large-sized and heavy, it is mechanically difficult to perform focusing by moving the entire telephoto lens.
Accordingly, in the related art, there are many telephoto lenses that perform focusing by moving some lens units. Among them, an inner focus type where focusing is performed by moving some central lens units of a relatively small and light optical system other than a front lens unit is used. In Japanese Patent Application Laid-Open No. 9-145996, Japanese Patent Application Laid-Open No. 8-327897, and Japanese Patent Application Laid-Open No. 2009-271354, each of which includes a first lens unit having positive refractive power and a second lens unit having negative refractive power in order from an object side, focusing is performed by moving the second lens unit to an image plane side along the optical axis.
In general, as the focal length of a telephoto lens becomes longer, an entire lens system becomes large-sized. Accordingly, in a telephoto lens, it is important to promote a small size of an entire lens system of a telephoto lens, and appropriately correct chromatic aberration in particular, among various aberrations occurring as the focal length becomes longer. In addition, it is important to perform focusing promptly with a small-sized and light lens unit other than a front lens unit, and by lessening a burden of a driving unit.
In general, as the focal length of a telephoto lens becomes longer, a front lens unit having positive refractive power in particular becomes large-sized and heavy. Accordingly, in a telephoto lens, it is important to appropriately set the configuration of lenses of a front lens unit having positive refractive power to promote a small size and a light weight of an entire photographic optical system, appropriately correct chromatic aberration, and obtain a high optical performance. If the configuration of lenses of the front lens unit is inappropriate, as the entire photographic optical system becomes large-sized and various aberrations increase, it becomes very difficult to obtain a high optical performance.